I know it's a rookie question, but it's ok, I'm a rookie at this. I've been trying to figure this out from pictures on different dealer sites but I can't. What is the difference between an M35 and an M40 helmet? And am I correct that the difference on the M42 is the flare or lip on the bottom edge of the helmet?

Also, out of all the books on headgear and/or helmets that are listed on this site, which one would you guys recommend as the best to educate the beginner?

I would recommend Baers book "The History of the German Steel Helmet 1916-1945". But a good forum will keep you up to date and recommend that.
The M35 had a seperate piece for the vent bushing whereas the M40 had a stamped vent that is part of the shell..

Another point to make things more confusing...the Germans never referred to the helmet as anything but M35. M40 and M42 are collector terms used to describe the manufacturing variances in the M35 design as the war progressed. Brits often refer to the M42 (pressed vent bushings and raw, flared skirt edge) as M43

Thanks Chris! Now is it just me, or the photos, or maybe the size difference between the helmets? the M40 and 42 look deeper (or taller) than the M35. Almost more like the WW1 helmets. That is not the case in reality, correct?

It's just you, plus the size of the helmets and the way the photos were taken... ; in fact, it's sometimes hard to tell if a helmet is a M35 or a M40 from period pics, and even impossible if it wears a second paint layer or from the distance...
But if you look closely, you will notice that M.40s have a more round shape than M.35s : the forms used in the presses were changed for this model.